[Polly] [PPCGCodeGeneration] Add managed memory support to GPU code

generation.

This needs changes to GPURuntime to expose synchronization between host
and device.

1. Needs better function naming, I want a better name than
"getOrCreateManagedDeviceArray"

2. DeviceAllocations is used by both the managed memory and the
non-managed memory path. This exploits the fact that the two code paths
are never run together. I'm not sure if this is the best design decision

Reviewed by: PhilippSchaad

Tags: #polly

Differential Revision: https://reviews.llvm.org/D32215

llvm-svn: 301640